AP Bio - Unit 4: Cell Communication & the Cell Cycle

Signal Transduction

cell converts external signal into internal response, using relay of molecular events (w/ second messengers), amplifying/transmitting signal from cell surface to inside cell for cellular behavior response

Receptor

a specialized protein (on cell surface or inside cell), specifically binding to a signaling molecules (ligand), initiating a cellular response

Ligand

a signaling molecule that binds specifically to a receptor protein on a target cell, which initiates a signal transduction pathway that leads to a specific cellular response

Small, Hydrophobic Ligands

-internal (cytoplasmic) receptor

-diffuse directly through membrane

-ex: steroid hormones

Water-Soluble Ligands

-external (cell membrane) receptor

-cannot diffuse directly through membrane

-ex: peptide hormones, ions, proteins

Autocrine

a cell produces the ligand for its OWN receptor

Paracrine

ligands are received by NEARBY cells

Endocrine

ligands TRAVEL through the BLOODSTREAM

Juxtacrine

(cell-to-cell signaling) direct contact: plasmodesmata (plant) or gap junctions (animal)

-small molecules & ions move through channels

Hormone

chemical signals, different structure = different type

Plasmodesmata

juxtacrine signaling (direct contact) in a PLANT

Gap Junction

juxtacrine signaling (direct contact) in an ANIMAL

Intercellular

communication BETWEEN different cells

Intracellular

processes within a SINGLE cell, specifically the signal transduction pathways that relay a received external or internal signal from the cell membrane through the cytoplasm to the nucleus, often using second messengers

Phosphorylation

activates relay proteins as kinases add phosphates from ATP to another protein, leading to a phosphorylation cascade as one enzyme phosphorylates another in a chain reaction, amplifying the signal

Dephosphorylation

relay proteins are inactivated as phosphatases remove phosphates

Kinase

an enzyme that adds phosphates from ATP to another protein (acts as molecular switch to activate or deactivate them)

Phosphatase

remove phosphates in dephosphorylation, reversing the action of kinases, which is crucial for turning off singling pathways and regulating the cell cycle by activating or inactivating target proteins

Enzyme-Linked Receptor

-ligand activates receptor

-activation initiates signal transduction

-ex: receptor tyrosine kinase

G-Protein Coupled Receptor

-ligand binding activates the G protein (attached to receptor)

-activated G protein activates other proteins (signal transduction)

Ion Channel Linked Receptors (Ligand-Gated Ion Channels)

ligand binding opens channel (conformational change)

Second Messenger

assist with transduction as they relay & amplify the signal in the cell

-ex: molecules (cAMP, DAG, IP₃), ions (CA²⁺)

Positive Feedback

AMPLIFY the signal until the process is complete

-no return to baseline or set level

-achieve a process: birthing a child, producing milk, forming a blood clot, ripening fruit

Negative Feedback

STOP the signal

-return to a baseline or set level

homeostasis: blood sugar, body temperature, etc.

Cell Cycle

the series of events that occur between cell divisions

Interphase

-longest phase

-grow & prep for mitosis

G₁ (Growth/Gap) Phase 1

-growth, make more organelles

-metabolically active (normal cell functions)

-”decision”: divide or enter G₀

G₁/S Checkpoint

is the cell ready to divide?

check:

-DNA ok?

-cell size?

-enough factors?

-growth factors?

-sufficient ATP/resources?

G₀

stops the cell cycle

-normal functions

-non-proliferative (non-growing) cells stay here

-nerve cells, muscle cells

-exit with proper signals

-growth hormones

S (Synthesis) Phase

-DNA replication (chromatin)

-cell is committed to division

G₂ (Growth/Gap) Phase 2

-synthesize proteins needed for mitosis (like microtubules)

-make lots of ATP

-two centrosomes present (1 —> 2)

G₂/M Checkpoint

is the cell ready for mitosis?

check:

-DNA ok?

-replication complete?

Mitosis

NUCLEAR DIVISION that results in TWO IDENTICAL nuclei

Prophase

-nucleolus & nuclear membrane disappear

-chromatin condenses to duplicated chromosomes

-centrosomes move to opposite poles

-spindle apparatus forms

Metaphase

-spindle fibers align chromosomes at the equator of the cell (metaphase or equatorial plate)

M (spindle) Checkpoint

is the cell ready for anaphase?

check:

-are the chromosomes properly lined up?

Anaphase

-sister chromatids are separated:

-motor proteins shorten kinetochore microtubules & pull chromatids to either pole

-polar microtubules lengthen and push

Telophase

-spindle disappears

-chromosomes unwind into chromatin

-nucleolus & nuclear membrane reform

-cytoplasm begins to divide

Cytokinesis (Animal Cells)

division of the cytoplasm:

animal cells:

-cleavage furrow deepens

contractile ring (band of actin filaments) tightens

-2 daughter cells form

Cytokinesis

division of the cytoplasm:

plant cells

-cell plate is formed by vesicles containing cellulose

-vesicles fuse to form the cell wall

Proliferative

an increase in cell number through cell division, which is crucial for growth and repair, but can leader to cancer when uncontrolled (faulty checkpoints, constant growth factor signals)

Non-Proliferative

condition of cell NOT dividing or undergoing the cell cycle, and typically exit the active cell cycle (mitosis) and enter G₀

Chromosome

highly condensed structure of DNA tightly coiled around histone proteins carrying genetic information, become visible during cell division (mitosis - prophase) to ensure proper DNA distribution (chromosomes easier to split than a bundle of chromatin)

-consists of two identical sister chromatids joined at a centromere after DNA replication

Chromatid

one of two identical halves of a duplicated chromosome, joined at the centromere, called chromatids, and they separate during cell division to become individual chromosomes in daughter cells

Spindle Apparatus

crucial network of microtubules and proteins that forms during cell division to organize, align, and separate chromosomes into sister chromatids into two new daughter cells, ensuring accurate chromosome distribution and genetic stability

-composed of spindle fibers (microtubules) extending from centrosomes to kinetochores on chromosomes, pulling them apart to opposite poles

Cyclin

positive regulators (advance cell cycle), and are made as needed & broken down after use (levels fluctuate), as they bind to and activate cyclin-dependent kinases to trigger cell cycle progression and checkpoints

Cyclin-Dependent Kinase

positive regulators (advance cell cycle), and are kept at constant levels, as they regulate cell cycle progression by phosphorylating target proteins, but are only active when bound to cyclin

Tumor Suppressor

negative regulator proteins (inhibit cyclin/CDK pairs, stopping the cell cycle), and are normal genes that halt division or trigger apoptosis to prevent uncontrolled proliferation and tumor formation (ex: p53)

-if mutated/inactivated, it loses its protective function, allowing cells to divide excessively and potentially become cancerous

Proto-Oncogene

normal gene that promotes healthy cell growth, division, and survival

-becomes oncogene when mutated or overactivated

Oncogene

mutated proto-oncogene (normal gene for cell growth) that becomes permanently overactive, promoting excessive, uncontrolled cell division and potentially causing cancer

Cancer

uncontrolled cell division of cells with damaged DNA

Checkpoint

critical point in cell cycle where regulatory proteins assess internal and external conditions, deciding whether the cell should proceed, pause for repair (enter G₀), or trigger programmed cell death (apoptosis)

Mutation

a permanent change in the DNA sequence, altering the genetic code which can change the protein produced, impacting cell signaling, function, or even leading to disease

-single base changes (point mutations) to large chromosomal rearrangements

-when multiple mutations accumulate in genes for positive and negative regulators, uncontrolled cell division may result

-mutations that inactivate p53 are common in many types of cancer

Apoptosis

programmed cell death, a controlled process where a cell self-destructs to eliminate old, damaged, or unnecessary cells, crucial for development and maintaining tissue health